Displaying Microsoft Exchange Server attachment in PHP

Question

I've searched stackoverflow and haven't found anything quite like what I'm having trouble with. I have a .NET app that is pulling data from Microsoft Exchange Server and feeding it into a PHP app via a curl request. I can read the e-mail just fine, and I can retrieve attachment data just fine (or so I think). The problem is that the attachment data doesn't seem usable. It comes in the following format:

{"<string #1>":
    {"Content":"<very, very long string>",
     "ContentId":null,
     "ContentLocation":null,
     "ContentType":null,
     "Id":"<string #1 repeated>",
     "Name":"Picture (Device Independent Bitmap)",
     "Filename:"null
    }
}

This is just one example. I have a few that are jpegs, pdfs, etc. ContentID, ContentLocation, ContentType, and Filename are always null, probably due to the ancient version of Exchange being run. Content is a string that can easily be several hundred thousand characters long. I've tried displaying it in an image tag, and I've tried base64_decoding it, but it only ever comes up with garbage. Decoders online have only come up with garbage as well. What data do I need to use to display this image? Or is there a specific set of manipulations I need to do on the string to get it to be human-readable?

I'd be happy to include the string, but as I said: it's a very lengthy string.